Ketenes can be perceived as inner anhydrides of carboxylic acids. Most ketenes dimerise or are stable only as dimers, such as the ketenes which derive from higher fatty acids.
Generally, these ketene dimers are prepared by reacting corresponding carboxylic acid chlorides with tertiary amines, more particularly triethyl amine. The course of reaction, as represented for example by the reaction of lauric acid chloride with triethyl amine, may be depicted as follows. EQU 2C.sub.10 H.sub.21 --CH.sub.2 --COCl+2(C.sub.2 H.sub.5).sub.3 N EQU 2(C.sub.2 H.sub.5).sub.3 N.HCl+2[C.sub.10 H.sub.21 --CH.dbd.C.dbd.O] EQU C.sub.10 H.sub.21 --CH.dbd.C--O EQU C.sub.10 H.sub.21 --CH--C.dbd.O EQU 2(C.sub.2 H.sub.5).sub.3 N.HCl+2NaOH.fwdarw.2(C.sub.2 H.sub.5).sub.3 N+2NaCl+2H.sub.2 O
The intermediarily formed ketene immediately dimerises into a diketene. The amine can be recovered from the amine hydrochloride formed as by-product by mixing an aqueous solution of the hydrochloride with sodium hydroxide solution and separating the organic phase which forms.
During the mixing of triethyl amine and fatty acid chlorides the viscosity rises very rapidly to high values because the crystalline growth of the triethyl amine hydrochloride precipitating under reaction conditions which is above all a dendritic growth, and prevents active stirring and hence the removal of the released reaction heat. Thus, to reduce and control the viscosity it was necessary up till now to carry through the conversion in an inert organic solvent.
U.S. Pat. No. 2,369,919 describes a process in which comparatively strongly diluted fatty acid chloride dissolved in an anhydrous solvent such as benzene or ether, is presented and mixed with the tertiary amine. The amine hydrochloride formed precipitates and must be separated by suction from the organic solution in which it is present as a very particulate suspension. The drawback to this process is that working with organic solvents, the recovery of which is expensive and time-consuming; moreover, working with organic solvents is always attended with hazards for fellow workers and the environment. Also, losses of solvents and product will occur, for instance when working up the filter cake.
DE-OS 2 327 988 discloses a process in which the amine hydrochloride is separated by briefly washing it with a diluted solution of neutral salt, e.g. with a 10% solution of sodium sulphate. However, in this process also a water-immiscible organic solvent is employed. Besides, using sodium sulphate means the introduction into the process of a further chemical substance, which likewise must be worked up again or disposed of.
Mixing the suspension with carboxylic acids such as formic acid, acetic acid, and the like, as can be inferred from DE-OS 2 35 488, is attended with similar drawbacks.
Frequently, the purity of ketenes obtained in this way is not satisfactory; thus DE-OS 2 927 118 recommends carrying out the conversion of the fatty acid chloride with a mixture of trimethyl amine and some other tertiary amine. Although the use of two tertiary amines leads to a partial reduction of the reaction viscosity, nevertheless this process also requires inert solvents to control the viscosity of the intermediate product. Hence, this process is also attended with the drawbacks already mentioned above because use has to be made of organic solvents.
Finally, DE-OS 3 434 212 does not describe an essentially different process either, since instead of the otherwise used organic solvent it employs melted wax, which, ultimately, is likewise an organic solvent. Admittedly, this wax does not have to be removed, since it can be used together with the diketene in special paper sizing processes. However, it is not possible to prepare diketenes in the pure form according to this process.
Although many processes for the preparation of long-chain ketene dimers are already known, the need for enhanced preparative processes remains.
It is therefore the object of the invention to provide a process which is not affected by the disadvantages outlined above and, in particular, works without the use of an organic solvent which must be recovered and purified, can be carried through in a commercially and technically advantageous manner, is environment-friendly, and does not yield any additional by-products which must be disposed of or will appear as impurities in the production waste water.